Permeability standard



March 13, 1956 M. R. GROSS 2,738,460

PERMEABILITY STANDARD Filed Aug. 8, 1952 I I y ymmlmmlmy x 2 INVENTOR 3g Mariz lz Robert GI mgfi $3 33 BY Q 3;; 50 I Q ATTORNEY United State O PERMEABILITY STANDARD Martin R. Gross, Annapolis, Md., assignor to Severn Engineering Co., Inc., Annapolis, Md., a corporation of Maryland Application August 8, 1952, Serial No. 303,207

7 Claims. (Cl. 324-34) vThis invention relates to permeability standards and more particularly to such devices for use with indicators for gauging the permeability of samples of material having very low permeability.

It is a general object of the present invention to provide novel and improved permeability standards for use with permeability indicators adapted to operate in the very low permeability range.

More particularly it is an object of the present invention to provide permeability standard units each adjusted to a predetermined low mg and arranged to be interchangeable in permeability indicators of the type defined.

An important object of the invention resides in the provision of a permeability standard unit in the form of an interchangeable button having a head and stem of non-magnetic material housing a soft iron'plug adjusted to be spaced by an air gap from a magnet-stopping plane surface such a distance as to provide an effective predetermined low m value.

A prominent feature of the device is the provision for adjusting a soft iron plug toward and from a magnet pole stop-surface to predetermine the effective m of the device, by threading the plug exterior and the interior of the hole in a non-magnetic housing mounting the same.

Other and further objects and features of the invention will become apparent to those skilled in the art upon a consideration of the accompanying drawing and following specification wherein is disclosed a single exemplary embodiment of the invention with the understanding that such changes and modifications may be made therein as fall within the scope of the attached claims without departing from the spirit of the invention.

In said drawings:

Fig. 1 is a perspective view looking from beneath, of a low m permeability indicator of the type described, interchangeably fitted with a permeability standard unit constructed in accordance with the present invention;

Fig. 2 is a side elevation thereof partially in vertical longitudinal central section illustrating the relationship of the test magnet and the permeability standard unit;

Fig. 3 is a longitudinal central section, on an enlarged scale, of a permeability standard unit in accordance with the present invention; and

Fig. 4 is a top plan view thereof illustrating the indicia thereon designating the permeability.

In my copending patent application for Indicator for Low Permeability Materials, Serial No. 284,481, filed April 25, 1952, there is disclosed a permeability indicator or tester for operation in the low permeability range and adapted to work with objects or samples of varying shapes and sizes to indicate with moderate accuracy the permeability thereof with maximum facility and convenience. Figs. 1 and 2 of the drawings attached hereto illustrate in detail the construction of a commercial embodiment of such a tester. The working parts are enclosed in a housing, preferably of transparent plastic, including bottom 2,738,460 Patented Mar. 13, 1956 10, side walls 11, 12 of channel form, end plates 13 and removable cover 14.

A balanced beam 15 comprising a rod threaded at both ends is fitted with central transverse pivots 16 whose pointed ends are fulcrumed in cupped screws 17 threaded through walls of the housing and fitted with lock nuts as shown. On the right end of the beam is a plastic collar 18 radially threaded to receive the end of the beam and vertically bored for a close fit for the bar magnet 20, which is locked therein by engagement by the end of the beam so that its lower portion projects through an opening 21 in the botom wall of the housing. This permanent magnet is preferably of high magnetic coercive force and moderately high magnetic moment. The ends of the magnet are carefully ground to uniform hemispherical shape and identical radius of curvature, to provide equal contact and active areas for engagement with flat surfaces.

The magnet and its collar are balanced by means of the counterweight 22 adjustable along the threads at the opposite end of the beam and fixed in adjustment by lock nuts as shown.

The cover of the housing is provided with a large threaded aperture 25, opposite the opening 21, to receive the threaded stem of the permeability standard unit 26 in axial alignment with the magnet 20. When the upper pole of the magnet engages the inner end of the unit the lower pole projects below the housing bottom for engagement with the sample material being tested. If this material is of higher permeability than that of the standard unit it will pull the magnet away from the unit as the tester is lifted after contacting the magnet with the sample. If the magnet pulls away from the sample it indicates a lower permeability than that of the test unit.

Test units may be provided each having a different value of permeability in order that the m of the sample may be ascertained by successive tests as will be understood.

Permeability standards for low mus are difficult to establish and in accordance with prior endeavors resulted from the heat treatment of various alloys, such for instance as a -40 copper-nickel plus l /2% iron alloy. Permeability is determined bythe type and degree of heat treatment and to obtain a predetermined value was exceedingly difficult and achieved only by constant cut and try methods. The permeability standard unit of the present invention obviates these difficulties and makes use of a simple soft iron plug and adjusting means whereby the permeability is the resultant of that of the iron plus that of an adjustable air gap.

Referring to Fig. 3, the test unit 26 is made up from a suitable housing of non-magnetic material, conveniently one of the satisfactory plastics and into the form of a plug having a cylindrical head and a reduced diameter threaded stem 27. This stern threads into the bore 25 in the cover of the instrument housing and the under surface 28 of the head stops against the top of the housing cover to determine the distance that the fiat under surface 30 of the plug stem extends below the cover. The housing 26 is longitudinally bored and internally threaded as at 31. This bore may stop just short of the stem end to provide a fiat surface normal to the stem axis against which the upper pole of the magnet is stopped when attracted to the unit, but since plastics are relatively soft continued use may provide suflicient indentation to destroy the careful calibration of the instrument, and for this reason it is preferred to provide an insert 32, set in a counterbore at the lower end of the stem, and formed of some hard non-magnetic material such as stainless steel. This can be quite thin, as shown, to leave a substantial air gap 33 between it and the lower end of 3 the'soft iron plug 34 externally threaded to fit in the bore 31' and provided 'with a-kerf35- to receive the-adjusting blade of a non-magnetic screwdriver.

The desired permeability value of the test unit, in respect toamagnet'identical in sizeand axialdisposi'tion to the magnet in the tester, is adjnsted 'with'the aid of a'suitable'perm'eameter by rotation of the plug 34' so that the efi'ective'pe'rmeability'isthat of thecombination of the air gap and the ironplug. When the predetermined low permeability has been attained suitable sealing means i's'used to "secure'the plug in position and a cover 36 applied-over the'top of theheadof the plug unit. This may-conveniently be of a contrasting color representative of the muvalue oftheunit, and as seen in Fig.4 should alsohave indicia thereon designating the value.

'With a plurality of therpermeability standard units varying'in value by small amounts, it is possible to bracket the permeability of samplesbeing tested. If two instruments are used with dilferent permeability standards in them theeffect is that of a go or no go gauge for materials having a mu intermediate to that of the two standard units in the instruments.

The standards of the present invention are simple to make, calibrate and use and are equally as efiective for the purpose as those made by more tedious and expensive methods and with more costly material.

"I claim:

1. Apermeability standard unit for use in a low mu permeability indicator having a bar magnet mounted at one' end of' a balanced beam forcomparing'the attractionof opposite poles of themagnet for said unit and material being tested, said unit comprising a housing of non-magnetic material'having a flat area for contact with a pole 'of said magnet, a soft iron plug in said housing, and means mountingsaid plug, for adjustmenttoward and from said area, said plugbeing of'sufficient thickness to prevent saturation thereof by the field of said" magnet.

2. A permeability standard unit for use in a low mu permeability indicator having a bar magnet mounted atone end of a balanced beam for comparing the attraction of opposite'poles'of'the magnet forsaid unit and material being tested, said unit comprising a housing of non-magnetic material having a fiat area for contact with a pole of said magnet, a soft iron plug having a flat end in said housing and means mounting said plug with said end parallel to and spaced from'said area such' a distance as-to simulate the effect of a solid mass of paramagnetic material of a predetermined lower mu.

3. A permeability'comparison standard unit for interhavinga bar'magnet mounted at one-end of a balanced beam, and normal to the longitudinal axis and at right angles to the pivotal axis thereof, for directly comparing the attractions of the poles of said magnet for said unit and material being tested, said unit comprising a nonmagnetic housing having means for mounting the same in a fixed position relative to said magnet, one end of said housing having a non-magnetic thin wall having a fiat surface normal to and intersected by the magnet axis when the housing ismounted for engagement by one pole of the magnet, a bore in said housing normal and extending to said wall, a soft iron plug in said bore and having its lower end parallel to said surface, said plug being adjusted in positionrelative to said surface to have a predetermined mu effect on the pole of said magnet engaging said surface and means closing said bore above said plug.

4. The comparison unitas defined in claim 3 in which the walls'of said bore are threaded and the plug 'iscylindrical and threaded for adjustment in the bore.

5. The comparison unit as defined in claim'3 in' which the closure for'the bore above the plug is provided with indicia-to designate the mu ofthe unit.

6. A permeability comparison standard unit comprising in combination, a housing of non-magnetic material having a thick cylindrical head portion and a coaxial exteriorly threaded stemportion, said portions having a central longitudinal threaded bore, a relatively'thin closure offnon-magnetic material for the stem end of the bore, and a'cylindrical threaded plug of'paramagnetic material of'l'ow-retentivity threaded in said here and adjusted from the outer surface'of said closure to provide a'predetermined mu at said surface.

"TITheunit as'd'efined in claim 6 in which the plug is sealed in itsadjustedposition, and a closure forthe' upper end of said bore, said closure being inscribed with in'dic'ia-designat-ing the mu value of the unit.

ReEerences'Cited in the file of this'patent UNITED STATES PATENTS 1,746,627 Babbitt Feb. 11, 1930 1,778,655 Wyman Oct. 14, 1930 1,925,904 Mayne Sept. 5, 1933 2,338,732 Nosker Jan. 11, 1944 2,384,529 Breitenstein Sept. 11, 1945 2,504,778 Young Apr. 18,.1 '0

FOREIGN PATENTS 575,150 Great Britain Feb. 5, 1946 

6. A PERMEABILITY COMPARISON STANDARD UNIT COMPRISING IN COMBINATION, A HOUSING OF NON-MAGNETIC MATERIAL HAVING A THICK CYLINDRICAL HEAD PORTION AND A COAXIAL EXTERIORLY THREADED STEM PORTION, SAID PORTIONS HAVING A CENTRAL LONGITUDINAL THREADED BORE, A RELATIVELY THIN CLOSURE OF NON-MAGNETIC THREADED PLUG OF PARAMAGNETIC MATERIAL AND A CYLINDRICAL THREADED PLUG OF PARAMAGNETIC MATERIAL OF LOW RETENTIVITY THREADED IN SAID BORE AND ADJUSTED FROM THE OUTER SURFACE OF SAID CLOSURE TO PROVIDE A PREDETERMINED MU AT SAID SURFACE. 